Re: Fwd: Announcing the first stable release of guile-for-loops

[Linus Björnstam]

Yeah, when iterating through many different things at the same time it is 
extremely helpful. 

I would have loved syntax parse when writing the macros :) I read your for 
loops code in awe, at least until I saw how you "cheated" with set! :-P

My only chance of getting it into guile proper would be to 1. Make a SRFI and 
survive the SRFI process with my honour intact and 2. Code cleanup. Most of 
for/emit is OKish, but for/foldr needs to be beaten with a stick, burnt and 
rewritten. "bending hygiene" doesn't quite cover what I did to make it work. In 
the end I sort of kind of worked around it by making an API change, but it 
still stinks. Defining a new loop using for/foldr involves having to do a 
syntax->datum->syntax. No fun. I could check how racket does it, but given they 
can do syntax-local-introduce (which is specific to their "hygiene as sets of 
scopes") I suspect I am out of luck. 

Väl mött
  Linus Björnstam

On Fri, 24 Jan 2020, at 13:13, Stefan Israelsson Tampe wrote:
> 
> 
> ---------- Forwarded message ---------
> From: *Stefan Israelsson Tampe* <address@hidden>
> Date: Fri, Jan 24, 2020 at 12:42 PM
> Subject: Re: Announcing the first stable release of guile-for-loops
> To: Linus Björnstam <address@hidden>
> 
> 
> Would be cool to have those implemented in guile, that would make my 
> guile-syntax-parse a bit leaner
> 
> Regards
> Stefan
> 
> On Thu, Jan 23, 2020 at 3:03 PM Linus Björnstam 
> <address@hidden> wrote:
> > Hiya everybody!
> > 
> >  I have spent some time implementing efficient for loops for guile, and 
> > they are baked and ready to go. I have worked the last weeks at 
> > implementing generalized support for non-tail-recursive loops and am happy 
> > to announce for/foldr. It is a generic right fold, with support for 
> > delaying it's arguments as either thunks or promises. 
> > 
> >  The syntax is more or less the same as racket's loops, and they are 
> > generally compatible. The code generated is for almost all cases as fast as 
> > hand-rolled code. They are all expressed as left or right folds, and are as 
> > such (apart from for/list, but read about that in the documentation) free 
> > of mutation. They are all converted to named lets. 
> > 
> >  Some examples:
> > 
> >  (for/list ((a (in-range 1 6)))
> >  (* a a)) ;; => (1 4 9 16 25)
> > 
> >  (for*/list ((a (in-string "ab")) (b (in-range 1 3)))
> >  (list a b)) 
> >  ;; => ((#\a 1) (#\a 2) (#\b 1) (#\b 2))
> > 
> >  There are many more looping constructs, among others: 
> >  for/sum, for/vector, for/or, for/and, for/first, for/last and a 
> > side-effecting simple for.
> > 
> >  Here is a sieve of erathostenes:
> > 
> >  (define (erathostenes n)
> >  (define vec (make-vector n #t))
> >  (for/list ([i (in-range 2 n)] #:when (vector-ref vec i))
> >  (for ([j (in-range/incr (* 2 i) n i)])
> >  (vector-set! vec j #f))
> >  i))
> > 
> >  The code and documentation is available here: 
> > https://hg.sr.ht/~bjoli/guile-for-loops
> > 
> >  A web-friendly documentation can be found here: 
> > https://man.sr.ht/%7Ebjoli/for-loops-docs/for-loops.md 
> > <https://man.sr.ht/~bjoli/for-loops-docs/for-loops.md>
> > 
> >  The thing I had been waiting for is right fold. That allows us to write 
> > loops like guile's map: non-tail recursive:
> >  (for/foldr ((identity '())) ((a (in-list '(1 2 3))))
> >  (cons (* a a) identity))
> > 
> >  becomes equivalent to:
> > 
> >  (let loop ((random-identifier '(1 2 3)))
> >  (if (null? random-identifier)
> >  '()
> >  (let ((a (car random-identifier)))
> >  (cons (* a a) (loop (cdr random-identifier))))))
> > 
> >  Happy hacking
> >  Linus Björnstam
> >